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| United States Patent |
5,312,606
|
|
Boffito
, et al.
|
May 17, 1994
|
Process for the sorption of residual gas by means of a non-evaporated
barium getter alloy
Abstract
The process of the present invention provides for the sorption of unwanted
gas in a vessel by means of a non-evaporated barium getter. It comprises
the steps of reducing an alloy of Ba.sub.1-x A.sub.x Li.sub.4-y B.sub.y to
a particle size of less than 5 mm, under vacuum or an inert gas atmosphere
and then placing the reduced alloy in the vessel. Upon exposing the
reduced alloy to the residual gas in the vessel at a temperature of less
than 150.degree. C., the gas is sorbed. The metal A is a metal chosen from
the group consisting of elements of Group IIa of the periodic table of
elements, excluding barium. The metal B is chosen from the group
consisting of elements of Group IIIa of the periodic table of elements.
Preferably 0.ltoreq.x.ltoreq.0.5 and 0.ltoreq.y.ltoreq.3.5.
| Inventors:
|
Boffito; Claudio (Milan, IT);
Schiabel; Antonio (Milan, IT)
|
| Assignee:
|
SAES Getters SpA (Milan, IT)
|
| Appl. No.:
|
089620 |
| Filed:
|
July 12, 1993 |
Foreign Application Priority Data
| Apr 16, 1991[IT] | MI91 A 001038 |
| Current U.S. Class: |
423/210; 252/181.4; 252/181.7; 313/481; 313/561; 423/219; 423/235; 423/239.1; 423/247 |
| Intern'l Class: |
B01J 008/06 |
| Field of Search: |
423/239.1,219,210,247,235
252/181.4,181.7
313/481,561
|
References Cited
U.S. Patent Documents
| 859021 | Jul., 1907 | Soddy | 252/181.
|
| 4203049 | May., 1980 | Kuus | 252/181.
|
| 4665343 | May., 1987 | Ferrario et al. | 252/181.
|
| Foreign Patent Documents |
| 62-057640 | Aug., 1987 | JP.
| |
Other References
Joseph Babor, Basic College Chemistry 2nd Ed, 1953, p. 256.
|
Primary Examiner: Lewis; Michael
Assistant Examiner: Lovern; Wendy
Attorney, Agent or Firm: Murphy; David R.
Parent Case Text
This application is a continuation of application Ser. No. 07/854,568,
filed Nov. 20, 1992, now abandoned.
Claims
What is claimed is:
1. A process for the sorption of residual gas in a vessel by a
non-evaporated barium getter comprising the steps of:
i) reducing an alloy of Ba.sub.1-x A.sub.x Li.sub.4-y B.sub.y to particles
under a vacuum or inert gas to produce a reduced alloy; and
ii) placing the reduced alloy in the vessel; and
iii) providing a partially evacuated atmosphere within the vessel to
produce an evacuated vessel; and
iv) exposing the reduced alloy to the residual gas at a temperature less
than 150.degree. C., and absorbing the residual gas without vaporizing the
reduced alloy wherein,
"A" is a metal chosen from the group consisting of elements of Group IIa of
the periodic table of elements, excluding barium, and,
"B" is a metal chosen from the group consisting of Group IIIa, of the
periodic table of elements and magnesium, and
"x" has a value from zero to 0.8; and
"y" has a value from zero to 3.5.
2. A process of claim 1 in which step iii) includes exposing the reduced
alloy to the residual gas at a temperature of between 50.degree. and
150.degree. C. for at least 10 minutes and then reducing the temperature
to less than 50.degree. C.
3. A process of claim 1 in which A is a metal chosen from the group
consisting of magnesium, calcium and strontium.
4. A process of claim 1 in which B is metal chosen from the group
consisting of boron, aluminium and magnesium.
5. A process of claim 1 in which 0.ltoreq..times..ltoreq.0.5.
6. A process of claim 1 in which the non-evaporated barium getter comprises
the alloy Bali.sub.4.
7. A process of claim 1 in which the non-evaporated barium getter comprises
the alloy Ba.sub.0.75 Ca.sub.0.25 Li.sub.4.
8. A process of claim 1 in which the non-evaporated barium getter comprises
the alloy Ba.sub.0.5 Ca.sub.0.5 Li.sub.4.
9. A process of claim 1 in which the non-evaporated barium getter comprises
the alloy BaLi.sub.3 Al.
10. A process of claim 1 in which the reduced alloy has a particle size of
less than 5 mm.
11. A process of claim 10 in which the reduced alloy has particle size of
less than 1 mm.
12. A process for the sorption of residual gas in a vessel by a
non-evaporated barium getter comprising the steps of:
i) comminuting an alloy of Ba.sub.1-x A.sub.x Li.sub.4-y B.sub.y to
particles in the absence of reactive gases to produce a particulate alloy;
and
ii) placing the particulate alloy in the vessel; and
iii) providing a partially evacuated atmosphere within the vessel to
produce an evacuated vessel; and
iv) exposing the particulate alloy to the residual gas at a temperature
less than 150.degree. C., and absorbing the residual gas without
vaporizing the particulate alloy wherein,
"A" is a metal selected from the group consisting of magnesium, calcium,
and strontium; and
"B" is a metal selected from the group consisting of boron, aluminum and
magnesium; and
"x" has a value from zero to 0.8; and
"y" has a value from zero to 3.5.
13. A process for the sorption of residual gas in a vessel by a
non-evaporated barium getter comprising the steps of:
i) comminuting an alloy of BaLi.sub.4 to particles in the absence of
reactive gas to produce a particulate alloy; and
ii) placing the particulate alloy in the vessel; and
iii) providing a partially evacuated atmosphere within the vessel to
produce an evacuated vessel; and
iv) exposing the particulate alloy to the residual gas at a temperature of
less than 150.degree. C., and absorbing the residual gas without
vaporizing the particulate alloy.
14. A process for the sorption of residual gas at temperatures less than
150.degree. C. in a closed vessel by contacting the residual gas with a
particulate alloy of Ba .sub.1-x A .sub.x Li .sub.4-y B .sub.y, wherein:
the particulate alloy has a particle size less than 5 mm; and sorption of
residual gas without vaporizing the particulate alloy; and
"A" is a metal selected from the group consisting of magnesium, calcium,
and strontium; and
"B" is a metal selected from the group consisting of boron, aluminum, and
magnesium; and
"x" has a value from zero to 0.8; and
"y" has a value from zero to 3.5.
Description
FIELD OF THE INVENTION
The present invention relates to a process for a sorbing residual gases by
means of a non-evaporated barium getter.
BACKGROUND
Barium getters are well known in the art. In the form of the more of less
pure element, barium was placed inside a metal container to protect it
from reaction with the atmosphere. Then, when required to be used, it was
mounted inside a vacuum device where, after partial evacuation and
seal-off of the device, the barium was caused to evaporate. The barium,
after evaporation, deposited in the form of a thin film within the vacuum
device where it sorbed the residual or unwanted gases throughout the life
of the device.
While these getter devices released barium they were also found to release
a large amount of undesirable gases that had been picked up during storage
or handling. This was due to the getter material being barium in the form
of an element which is reactive with gases.
In order to reduce the reactivity of the barium, it was then alloyed with
one or more metals. Such alloys were inter alia Ba-Mg, Ba-Sr-Mg, Ba-Mg-Al.
See for example the book "Getterstoff und Ihre Anwendung in the
Hochvakuumtechnik" by M. Littmann, E Winter'sche Verlagshandlung, Leipzig
1939. One of the most successful was the alloy BaAl.sub.4 having a weight
percent of barium from 40 to 60 percent. Such an alloy is very inert and,
as with all inert barium alloys, it must be evaporated before it can sorb
gases. It can be caused to dissociate and release barium by means of
applying heat to the BaAl.sub.4 alloy alone but, more recently, it has
become widespread to mix the BaAl.sub.4 with an approximately equal weight
of nickel. These two materials, in powder form, when heated react
exothermically to form a solid residue of Ni-Al and evaporated Ba.
However, these getter materials have to be heated to about 800.degree. C.
before the exothermic reaction starts whereupon they reach 1000.degree.
C., and more, when there is the sudden release of heat on reacting
exothermically.
In Japanese Patent Publication number SHO 42-4123 the barium-aluminium
(about 50% Ba) alloy is mixed with, preferably, 15% by weight of powdered
tin to produce getters. Said getters are heated by means of high frequency
electrical induction to about 600.degree. C. for one minute during the
exhaust process. As a result of the reaction which may be produced by said
heating, it is considered that BaSn.sub.2 may be produced, or liberated
barium is produced, from the barium-aluminium alloy by reaction of
aluminium and tin. In either case, a mixed getter material of
barium-aluminium alloy and tin which is stable at a normal temperature is
activated and absorb gases at a normal temperature. Nevertheless there is
a heating process involved which requires temperatures of several hundreds
of degrees centigrade. Furthermore an uncontrolled chemical reaction is
taking place.
Another family of getter devices has been based upon the elements zirconium
or titanium. Powdered Zr 84%-Al 16%, Zr.sub.2 Fe nd Zr.sub.2 Ni are among
these. They are known as non-evaporated getters because they do not
require any of their component elements to be evaporated in order to
become capable of sorbing gas. However they do require heating to a high
temperature to make them gas sorptive. This is because they are covered
with surface layers of oxides and nitrides which passivate them and render
them inactive. Upon heating, in vacuum, the passivating layers diffuse
into the bulk material and the surface becomes clean and active. This
heating process usually takes place at a high temperature, say 900.degree.
C. for about 10-30 seconds. This temperature can be reduced but requires a
longer time. For instance several hours at 500.degree. C.
Even more recently non-evaporated getters based on Zr-V have been used.
Such alloys as Zr-V-Fe and Zr-V-Ni have gained widespread acceptance as
"low temperature" activatable non-evaporated getters. By low temperature
activatable it means that a significant proportion of their gettering
activity already becomes available within a relatively short time at
moderate temperatures. It is believed that this is due to the ease with
which the surface layers of passivating materials may diffuse into the
bulk material at these relatively low temperatures. Whatever the reason
for their ability to become active at these relatively low temperatures of
400.degree.-500.degree. C. this can still be an undesirably high
temperature under many circumstances. All these gas sorptive material have
been used in admixture with other materials, both gas sorptive or not, in
an attempt to lower their temperature of activation.
There are many occasions in which it is desirable to remove unwanted gases
from a vessel which under no circumstances can be allowed to be subject to
a high temperature. Such may be the case for instance when the vessel is
made of organic plastic or contains components of organic plastic. The
organic plastic may melt. Even if the organic plastics do not melt they
may reach such a temperature that they start to decompose or at least give
off a large amount of gas which may be hydrocarbons or other organic
gases. If they are sorbed by the getter material, this causes their
premature failure as they only have a finite gettering power or ability to
sorb a fixed quantity of gas. The rapid sorption of a large amount of gas
impares their ability to later sorb gas during the life of the device in
which they are employed. Otherwise there remains too high a gas pressure
for the device to work as intended. This temperature may be as low as
about 150.degree. C. At these temperatures, and lower, oxygen and water
vapour permeation, and especially nitrogen, can be a problem. Lithium
organic resins have been proposed for the sorption of gas impurities from
impure gas streams, but they are used for the purification of nitrogen gas
and not for its sorption, see U.S. Pat. No. A-4,603,148 and U.S. Pat. No.
A-4,604,270.
Although it has been suggested that non-evaporated getters can be
introduced into the device in a pre-activated form, that is when they have
already been heated to a high temperature of about 600.degree. C., they
have already been subjected to many manufacturing processes such as
grinding to fixed particle size, mixing with other materials, compaction
or forming into pellets.
BRIEF OBJECTS OF THE INVENTION
It is therefore an object of the present invention to provide a process for
the sorption of residual gas in a vessel which is free from one or more of
the disadvantages of prior art processes.
It is another object of the present invention to provide a process for the
sorption of residual gas in a vessel which does not require the getter
materials to be activated at temperatures greater than 150.degree. C.
A further object of the present invention to provide a process for the
sorption of unwanted gas in a vessel which does not require temperatures
of greater than 150.degree. C.
It is yet another object of the present invention to provide a process for
the sorption of residual gas in a vessel which does not require the getter
to be mixed with other materials. Another object of the present invention
to provide a process for the sorption of residual gas in a vessel which
can be used in vessels made of organic plastic.
Yet a further object of the present invention is to provide a process for
the sorption of nitrogen gas in a vessel made of organic plastic or which
contains organic plastic.
These and other objects and advantages of the present invention will become
clear to those skilled in the art by reference to the following
description thereof and drawings wherein:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a drawing showing in a schematic form an apparatus for measuring
the sorption properties of alloys useful in providing a process of the
present invention,
FIG. 2 shows the results of sorption tests of N.sub.2 at 25.degree. C.
performed on alloy of BaLi.sub.4 of the present invention,
FIG. 3 shows the results of sorption tests of various gases at 25.degree.
C. performed on an alloy of BaLi.sub.4 of the present invention,
FIG. 4 shows the results of sorption tests of N.sub.2, at various
temperatures, performed on an alloy of BaLi.sub.4 of the present
invention,
FIG. 5 shows the results of sorption tests of N.sub.2 at 25.degree. C.
performed on an alloy of BaLi.sub.4 of the present invention,
FIG. 6 shows the gas sorption speed derived from the curves of FIGS. 2-5 as
a function of the quantity of gas sorbed,
FIGS. 7-9 show the results of sorption tests of N.sub.2 at 25.degree. C.
performed on an alloy of Ba.sub.0.75 Ca.sub.0.25 Li.sub.4 of the present
invention.
FIG. 10 shows the gas sorption speed derived from the curves of FIGS. 7-9
as a function of the quantity of nitrogen sorbed,
FIGS. 11-15 show the results of sorption tests of N.sub.2 at 25.degree. C.
performed on an alloy of Ba.sub.0.5 Ca.sub.0.5 Li.sub.4 of the present
invention,
FIG. 16 shows the gas sorption speed derived from the curves of FIGS. 11-15
as a function of the quantity of nitrogen sorbed,
FIGS. 17-18 show the results of sorption tests of N.sub.2 at 25.degree. C.
performed on an alloy of BaLi.sub.3 Al of the present invention,
FIG. 19 shows the gas sorption speed derived from the curves of FIGS. 17-18
as a function of the quantity of nitrogen sorbed; and
FIG. 20 shows the gas sorption speed of BaLi.sub.4 and a traditional
non-evaporated getter, for nitrogen, at various temperatures obtained by a
different technique to that used for obtaining curves of FIGS. 6, 10, 16
and 19.
DETAILED DESCRIPTION OF THE INVENTION
The process of the present invention provides for the sorption of residual
gas in a vessel by means of a non-evaporated barium getter. It comprises
the steps of reducing an alloy of Ba.sub.1-x A.sub.x Li.sub.4-y B.sub.y,
preferably to a particle size of less than 5 mm, under vacuum or an inert
gas atmosphere and then placing the reduced alloy in the vessel. Upon
exposing the reduced alloy to the residual gas in the vessel at a
temperature of less than 150.degree. C. the gas is sorbed. The metal A is
a metal chosen from the group consisting of elements of Group IIa of the
periodic table of elements, excluding barium. The metal B is chosen from
the group consisting of elements of Group IIIa of the periodic table of
elements and magnesium.
The alloys of the present invention do not have to be activated, that is
they are already capable of sorbing gases at room temperature and
furthermore they do not have to be evaporated to produce a film of active
material, such as barium before they sorb gas. Nevertheless they can be
activated by heating to a temperature of not greater than 150.degree. C.
This could be between 50.degree. C. and 150.degree. C. for at least 10
minutes and then reducing the temperature to less than 50.degree. C. The
alloys can be described by the general formula Ba.sub.1-x A.sub.x
Li.sub.4-y B.sub.y, where A is a metal chosen from the group consisting of
elements of Group IIa of the periodic table of elements, excluding barium.
The numbering of the Group of elements is that adopted by The American
Chemical Society. Thus A can be calcium, magnesium and strontium but is
preferably calcium as calcium is only slightly less reactive than barium.
Magnesium and strontium are less preferred for their lower reactivity. The
value of x may be as low as zero such that there is no metal of Group IIa
present (except the barium). On the other hand it may be as high as 0.8.
Above about 0.5 the alloy begins to lose its ability to react at room
temperature with the residual gas at a sufficiently high sorption speed.
The element B is any metal chosen from the group consisting of elements
Group IIIa of the periodic table of elements and magnesium. All members of
Group IIIa can be adopted, for instance boron, but aluminium is preferred
as it is readily available, it has a relatively high melting point and is
extremely cheap, while gallium is liquid near ambient temperatures. Indium
has a low melting point and is more expensive then aluminium. Tl is highly
toxic. Furthermore 0.ltoreq.y.ltoreq.3.5.
These compounds can be easily comminuted or reduced to a particulate form
without any difficulty. For instance they can be reduced to a particle
size of less than 5 mm in diameter, and preferably less than 1 mm, by
known techniques under a vacuum or inert atmosphere and then transferred
to the vessel containing the unwanted gas which is desired to be removed.
This is accomplished by placing the reduced alloy in the vessel and
exposing the reduced alloy to the residual gas at room temperature.
The reduced alloy can be transferred to the vessel immediately but
preferably takes place by means of an intermediate vessel in which the
alloy is stored under vacuum or an inert atmosphere until it is required.
Surprisingly they immediately start to sorb large amounts of unwanted gas.
Even more surprisingly they sorb nitrogen with a much higher sorption
speed and sorb greater quantities than prior art non-evaporated getter
devices after no activation, or an activation at not greater than
150.degree. C.
The invention may be better understood by reference to the following
examples wherein all parts and percentages are by weight unless otherwise
indicated. These examples are designed to teach those skilled in the art
how to practice the present invention and represent the best mode
presently known for practicing the invention.
EXAMPLE 1
This example is not representative of the present invention but is designed
to show an apparatus suitable for measuring the gas sorption properties of
alloys suitable for practicing processes of the present invention. FIG. 1
is a drawing showing in a schematic form an apparatus 100 for measuring
the sorptive properties of Ba.sub.1-x A.sub.x Li.sub.4-y B.sub.y alloys
useful in the present invention. A vacuum pumping system 102 is connected
by means of a first valve 104 to a dosing volume 106. Connected with
dosing volume 106 there is a series of second valves 110, 110', 110" for
the inlet of test gases from a series of test gas reservoirs 112, 112',
112", containing N.sub.2, artificial air, and CO respectively, and a
pressure measuring gauge 114. To dosing volume 106 is also connected, by
third valve 116, a test chamber 118 containing the sample 120 under test.
In operation valves 110, 110', 110" and 116 are closed and 104 is opened
and the vacuum pump system 102 pumped the system down to 10.sup.-6 mbar.
For all test the dosing volume 106 was a volume of 0.71 litre. A sample of
powered alloy 120, contained within a glass bulb test chamber 118 of
approximately 0.1-0.3 litre volume (depending upon the sample), under an
inert atmosphere of argon gas was attached to apparatus 100 via valve 116
(closed). Valve 116 was opened and again the system was pumped down to
10.sup.-6 mbar while the sample was held at about 100.degree. C. for 20
minutes which simulates a process to which the getter may be subject.
Valves 104 and 116 were then closed and test gas was admitted to dosing
volume 106, from gas reservoir 112, (112', 112") by opening valve 110
(110', 110") for a short while. The pressure was noted on pressure gauge
114, and was arranged to be such that the pressure was about 0.4-1.0 mbar
after opening valve 116 to introduce a dose of test gas to the sample 120.
EXAMPLE 2
This example was designed to show how to manufacture an alloy useful in the
process of the present invention.
In an iron crucible were placed 155.88 g of commercial grade barium (purity
greater than 98%) obtained from Degussa together with 11.3 g of lithium.
The crucible was placed in an induction furnace and heated under an argon
atmosphere at 400 mbar pressure with medium frequency induction heating
until the mixture was thoroughly melted and homogeneous thus forming a
fusion. The fusion was then poured into a cold copper mould and allowed to
cool to room temperature while still under the protective atmosphere of
argon.
The alloy corresponds to the intermetallic compound BaLi.sub.4.
EXAMPLE 3
This example was designed to show the use of an alloy in the process of the
present invention.
A barium-lithium alloy as prepare in Example 2 above was placed in a
glove-box under a protective atmosphere of argon at slightly greater than
1 atmosphere pressure. The alloy was ground using a mortar and pestle to a
particle size of less than 1 mm and a sample of 2.7 g was sealed in a
glass vessel of volume 0.17 litre. The sample in the glass vessel was then
attached as test chamber 118 to the test apparatus of Example 1. The
procedure of Example 1 was followed and a first dose of gas, in this case
nitrogen (from reservoir 112 by means of valve 110), was introduced to the
sample. The pressure in the vessel was measured by means of pressure gauge
114 as a function of time. The curve obtained is reported on FIG. 2 as
curve 1. The sorption was so rapid that the pressure drop was almost
instantaneous. A second dose of nitrogen was introduced and the pressure
in the vessel as measured by the pressure gauge 114 was reported as curve
2 on FIG. 2.
The gas was replaced by artificial air labeled "ARIA" in FIG. 3, that is a
mixture of 80% v/v N.sub.2 +20% v/v O.sub.2 (from reservoir 112' by means
of valve 110') and the pressure in the vessel was recorded as curve 3 on
FIG. 3. This was repeated to give curve 4. The gas was replaced by CO
(from reservoir 112" by means of valve 110") and the pressure in the
vessel was recorded as curve 5 for which the sorption was so rapid that
the pressure drop was almost instantaneous. Artificial air was again
introduced to give curve 6.
The sample was then cooled to 0.degree. C. and on introducing a sample of
N.sub.2 gas curve 7 was recorded.
The sample was then cooled to about -8.degree. C. to -9.degree. C. with a
bath of alcohol+iced water and on introducing a dose of nitrogen curve 8
was recorded.
Again at 25.degree. C. a dose of nitrogen was introduced and curve 9 was
recorded.
A further 3 doses of nitrogen were introduced to the sample and curves
10-12 were recorded.
The total doses of gas absorbed by the sample during these tests were,
N.sub.2 =2818 mbar.cm.sup.3 ; O.sub.2 =421 mbar.cm.sup.3 ; CO=216
mbar.cm.sup.3. FIG. 6 shows the gas sorption speed derived from the curves
of FIGS. 2-5, by differentiation, as a function of gas sorbed.
EXAMPLE 4
This example was designed to show how to manufacture another alloy, by
partial replacement of barium by calcium, useful in the process of the
present invention.
114.08 g of barium (type HP made by SAES Getters S.p.A.) was placed in an
iron crucible together with 30.75 g of lithium and 1.10 g of granulated
calcium (supplied by Carlo Erba, purity greater than 99.5%).
The crucible was placed in an induction furnace and heated under an argon
atmosphere of 400 mbar with medium frequency induction heating until the
mixture was thoroughly melted and homogeneous thus forming a fusion. The
fusion was then poured into a cold iron mould and allowed to cool to room
temperature while still under the protective atmosphere of argon. The
weight of alloy after fusion was 138 g.
The alloy corresponds to the composition Ba.sub.0.75 Ca.sub.0.25 Li.sub.4.
EXAMPLE 5
This example was designed to show the use of the alloy produced as in
Example 4 in the process of the present invention.
A barium-calcium-lithium alloy as prepared in Example 4 above was placed in
a glove box under a protective atmosphere of argon at slightly greater
than 1 mm atmosphere pressure. The alloy was ground to a particle size of
less than 1 mm with a pestle and a mortar and a sample of 2.0 g was sealed
in a glass vessel of volume 0.32 litre. The sorption properties were
measured as in Example 3 for N.sub.2 at 25.degree. C. and are reported as
curves 1-14 in FIGS. 7-9. FIG. 10 shows the nitrogen sorption speed
derived from the curves of FIGS. 7-9, by differentiation, as a function of
nitrogen sorbed.
EXAMPLE 6
This example was designed to show how to manufacture another alloy, by
replacement of more barium by calcium, useful in the process of the
present invention.
25.3 g of barium (type HP made by SAES Getters S.p.A.) was placed in an
iron crucible together with 7.38 g of granulated calcium (purity greater
than 99.5%) obtained from Carlo Erba and 10.23 g of lithium. The crucible
was placed in an induction furnace and heated under an argon atmosphere at
400 mbar pressure with medium frequency induction heating until the
mixture was thoroughly melted and homogeneous thus forming a fusion. The
fusion was then poured into a cold iron mold and allowed to cool to room
temperature while still under the protective atmosphere of argon. The
weight of alloy after fusion was 39.6 g. The alloy corresponds to a
composition Ba.sub.0.5 Ca.sub.0.5 Li.sub.4.
EXAMPLE 7
This example was designed to show the use of the alloy produced as in
Example 6 in the process of the present invention. A barium-calcium
lithium alloy as prepared in Example 6 above was placed in a glove box
under a protective atmosphere of argon at slightly greater than 1
atmosphere pressure. The alloy was ground to a particle size of less than
1 mm with a pestle of volume 0.15 a sample of 2.47 g was sealed in a glass
vessel of volume 0.15 litre. The sorption properties were measured for
nitrogen at 25.degree. C. as in Example 3, and are reported as curves 1-28
in FIGS. 11-15. FIG. 16 shows the gas sorption speed derived from the
curves of FIGS. 11-15 as a function of quantity of gas sorbed.
EXAMPLE 8
This example was designed to show how to manufacture yet another alloy
useful in the process of the present invention. 59.14 g of barium (type HP
made by SAES Getters S.p.A.) was placed in an iron crucible together with
8.97 g of lithium and 11.61 g of SAVA aluminium beads (purity greater than
98.5%). The crucible was placed in an induction furnace and heated under
an argon atmosphere of 400 mbar with medium frequency induction heating
until the mixture was thoroughly melted and homogeneous thus producing a
fusion. The fusion was then poured into a cold iron mold and allowed to
cool to room temperature while still under the protective atmosphere of
argon. The weight of alloy after fusion was 78.3 g.
The alloy corresponds to the composition BaLi.sub.3 Al.
EXAMPLE 9
This example was designed to show the use of the alloy produced as in
Example 8 in the process of the present invention. A
barium-lithium-aluminium alloy as prepared in Example 8 above was placed
in a glove box under a protective atmosphere of argon at slightly greater
than 1 atmosphere pressure. The alloy was ground to a particle size of
less than 1 mm with a pestle and a mortar and a sample of 2.4 g was sealed
in a glass vessel of volume 0.32 litre. The sorption properties were
measured as in Example 3 for N.sub.2 at 25.degree. C. and are reported as
curves 1-7 in FIGS. 17-18. FIG. 19 shows the gas sorption speed derived
from the curves of FIGS. 17-18, by differentiation, as a function of gas
sorbed.
EXAMPLE 10
This example was designed to show the N.sub.2 sorption properties of a
prior art alloy. A low temperature activatable non-evaporated of 70%
Zr-24.6% V-5.4% Fe (nominal weight composition) of weight 100 mg was
activated by heating to 450.degree. C. for 10 minutes under vacuum at
better than 10.sup.-3 mbar and then caused to sorb N.sub.2 at 25.degree.
C. at a pressure of about 10.sup.-5 mbar in an apparatus described in the
ASTM F798-82 Standard Practice for Determining Gettering Rate, Sorption
Capacity, and Gas Content of Nonevaporable Getters in the Molecular Flow
Region. The gettering rate (sometimes called sorption speed) was plotted
against the quantity sorbed, for the equivalent 2 g getter, to give curve
A on FIG. 20.
The test was repeated using a sorption temperature of 350.degree. C. and
the curve of nitrogen sorption, again for the equivalent 2 g getter was
plotted as curve B of FIG. 20.
EXAMPLE 11
This example was designed to show the N.sub.2 sorption properties of an
alloy according to the present invention. A barium-lithium alloy as
prepared in Example 2 above was prepared and 2 g, having a particle size
of less than 1 mm, was placed in the same ASTM apparatus used for Example
10. After being held at a temperature of 100.degree. C. for 15-20 minutes,
which simulates a process to which the getter may be subjected the getter
was cooled to 25.degree. C. and its curve of nitrogen sorption was plotted
as curve C on FIG. 20. After the getter had sorbed a quantity of gas the
flow was interrupted. On continuing, the curve C' was obtained, and
successively with curves C" and C"'.
EXAMPLE 12
The Example 11 was repeated except that the getter was activated at a
temperature of 130.degree. C. for 3 hours followed by 100.degree. C. for
16 hours and the getter was held at 80.degree. C. while sorbing nitrogen.
The gettering rate was again plotted against quantity sorbed to give curve
D on FIG. 20.
DISCUSSION
By comparison of curve C with curve A of FIG. 20, and D with B, it is seen
that alloys used in processes of the present invention have a higher
gettering rate (sorption speed) when they have sorbed a greater quantity
of nitrogen than traditional getter alloys. Furthermore these properties
are obtained after activation at much lower temperatures. In addition
FIGS. 6, 10, 16 and 19 show a remarkable ability for the large quantities
of N.sub.2 and other gases sorbed. Although the invention has been
described in considerable detail with reference to certain preferred
embodiments designed to teach those skilled in the art how best to
practice the invention, it will be realized that other modifications may
be employed without departing from the spirit and scope of the appended
claims. For instance, it will be realized that small amounts of lithium
can be replaced by other elements of the alkali metal series of elements,
and notably sodium and potassium without substantially affecting their
properties. Furthermore although the invention has been described and
exemplified in terms of the stoichiometric compound BaLi.sub.4 some degree
of departure from stoichiometry will be allowed within the formula
Ba.sub.1-x A.sub.x Li.sub.4-y B.sub.y, for example BaLi.sub.4 .+-.1Li.
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